In a PON system that is one of optical communication systems, continuous signals are first transmitted from a station-side optical line terminal (station-side OLT) to a subscriber's home optical network unit (subscriber-side ONU) arranged in each of houses, and the continuous signals are then received by the ONU. Because the ONU transmits a time-division multiplexed signal (packet) to the OLT, a receiver for the OLT is required to have a burst receiving function of instantaneously electrically regenerating a packet signal. At this time, the ONUs are arranged on various locations when seen from the OLT, and hence the OLT is required to receive packets different in optical power.
Meanwhile, as an element for converting the optical signal (packet) into a current in the OLT receiver, an avalanche photodiode (APD) is used. Because the APD has a function of multiplying a photocurrent, when a multiplication factor M of the APD is set to be high and an input signal has low optical input power, high receiving sensitivity can be realized. On the other hand, when an input signal has high optical input power and the multiplication factor is set to be high, there is such a risk that an overcurrent may flow to break down the APD.
Thus, in the PON system described above, the OLT may receive strong optical input light from the ONU, which is close to the OLT, and hence it is desired that the multiplication factor M be changed depending on each of input packets. Because the OLT is required to perform the reception in burst (convert optical signals having different intensities into electrical signals having uniform intensity), the multiplication factor M is required to be changed at high speed of the order of nanosecond (ns).
In Patent Literature 1, there is disclosed a circuit as an example of a related-art burst-mode receiver in which the multiplication factor M is changed at high speed as described above. The related-art burst-mode receiver, which is described as a bias control circuit for an APD, includes an avalanche photodiode (APD) for converting an optical signal into a current, a preamplifier unit for converting the generated photocurrent into a voltage to amplify the voltage, a current mirror unit capable of outputting a current having substantially the same value as the photocurrent of the APD, a high voltage generating circuit, a control circuit of the high voltage generating circuit, and a circuit for changing an output voltage of the control circuit.
The APD converts input light into a photocurrent. Then, in response to a signal obtained by converting a collector current of a transistor into a voltage by a resistor, the collector current serving as an output of the current mirror that exhibits a current having substantially the same value as the photocurrent of the APD, the APD changes a voltage input to the control circuit from the circuit for changing an output voltage of the control circuit, and further changes an output voltage of the high voltage generating circuit. In this manner, a voltage applied to the APD is changed so as to follow the photocurrent (input optical power) flowing through the APD (see FIG. 7 described later).